Groups of 6–39 monoclonal antibodies identifying 3–18 distinct epitopes on the nucleoprotein (NP), polymerase (P), hemagglutinin (H; equivalent in canine distemper and rinderpest viruses), and fusion (F) components of measles and canine distemper viruses were characterized in immunofluorescence tests with fixed Vero cell cultures infected with measles, canine distemper and rinderpest viruses. The majority of NP-specific monoclonal antibodies reacted with all three viruses, but one-third of the antibodies only reacted with the homologous virus. A few antibodies detected epitopes uniquely shared between either measles and rinderpest viruses or canine distemper and rinderpest viruses. Of the P-specific antibodies, two-thirds only reacted with the homologous virus, one antibody detected an epitope shared between canine distemper and rinderpest viruses, and the rest reacted with all three viruses. Also, the majority of antibodies against the H component were type-specific, but four antibodies reacted both with measles and rinderpest viruses. In contrast, the F component was antigenically highly conserved. 17 of 21 antibodies against this component reacted with all three viruses; one antibody reacted only with measles and rinderpest virus F components, and three antibodies reacted only with the homologous virus. No monoclonal antibody of any specificity selectively reacted with only measles and canine distemper viruses. Furthermore, the measles virus H component appeared to be more closely related to the equivalent rinderpest virus component than to the canine distemper virus component. Thus, it is proposed that rinderpest virus is the archevirus of the morbillivirus group from which canine distemper virus was first to evolve and, more recently (perhaps about 5,000 years ago), measles virus.